An optical waveguide device typically includes a substrate, a cladding layer formed on the substrate, and a waveguide—also called waveguide core—formed in the cladding layer for transmitting an optical signal. Such waveguide devices are also called integrated optical devices or opto-electronic integrated circuits. The core commonly has the shape of a strip or a rib. Light signals may be introduced into and out of the core by coupling optical fibers thereto. The core has a top surface and opposing parallel side walls. The core serves to confine an optical light signal transmitted thereto, and the light signal is mostly contained in the core as it is transmitted therethrough.
Complementary metal oxide semiconductor (CMOS) processes, such as masking and etching, are commonly used to form the core. Etching, such as reactive ion etching (RIE), can result in core side wall roughness that causes scattering of light propagating along the waveguide. This light scattering produces an optical loss in the waveguide, and can also lead to cross talk between adjacent waveguides. Isolation of waveguides is routinely accomplished by differences in index of refraction of the materials that make up the core and the cladding layer. However, differences in index of refraction between the core and a cladding material do not completely eliminate undesired communication between adjacent cores.